Explanation

History:

The United States is generally considered to have entered the conflict with the 7 December 1941 surprise attack on Pearl Harbor by the Empire of Japan and exited it with the 2 September 1945 surrender of Japan.

The Japanese attack on the US naval base in Pearl Harbor, Hawaii, led President Franklin Roosevelt to declare war on Japan.

A few days later, Nazi Germany declared war on the United States, and America entered World War II against the Axis powers.

***

World War II or the Second World War, was a global war that started from 1939 

The Little Boy type of bomb, was dropped on the Japanese city of Hiroshima on 6 August 1945 during World War II

The Fat Man was dropped on Nagasaki, Japan, on August 9, 1945, at 11:01 AM

the 2nd World War End on 1945